1. Field of the Invention
The present invention relates to a photosensitive resin composition for printing plates, and to a printing plate material comprising it. In particular, it relates to a water-developable, flexographic printing plate material.
2. Description of the Related Art
As having the advantage of elastomer characteristics, a photosensitive resin composition that comprises, as a carrier resin component, an elastomer of chlorinated rubber, styrene-butadiene block copolymer, polyurethane or the like and contains an ethylenically unsaturated compound and a photopolymerization initiator is useful as a flexographic printing plate material. Many proposals relating to it have been made, for example, as in U.S. Pat. No. 2,948,611, U.S. Pat. No. 3,024,180, JP-B-51-43374, etc.
Solid photographic resin materials for printing plates require development with halogenated hydrocarbons. It is desired to create water-developable, solid photographic resin materials for flexographic printing plates, and some proposals for them have been made.
In U.S. Pat. No. 4,517,279, U.S. Pat. No. 5,372,913 and U.S. Pat. No. 5,424,172, a method has been proposed which is to prepare a photosensitive resin composition containing a hydrophilic polymer such as carboxylated nitrile rubber and a hydrophobic polymer such as nitrile rubber. However, the composition proposed is problematic in that the aqueous developer for it must contain some additives, for example, like an aqueous alkaline developer or an aqueous, surfactant-containing developer, in order that the polymers in the composition could be dispersible in the developer.
In U.S. Pat. No. 4,275,142, U.S. Pat. No. 4,272,608, JP-A-51-63903, JP-A-53-10648 and JP-A-61-22339, a photosensitive resin composition comprising a carboxyl group-containing polymer has been proposed, in which the polymer is in the form of a metal salt or ammonium salt. However, the composition proposed is still problematic in that it is liquid, that its waterproofness and mechanical strength are unsatisfactory when used in printing plates, and that the aqueous developer for it must contain some additives, for example, like an aqueous alkaline developer or an aqueous, surfactant-containing developer.
For forming salt structures as in the above, another method has been proposed in U.S. Pat. No. 5,348,844, U.S. Pat. No. 5,350,661, U.S. Pat. No. 5,229,434 and JP-B-5-6178, which comprises forming ammonium salt structures of a carboxyl group-containing polymer of being in the form of latex or micro-gel particles dispersed in water. They say that the composition comprising the polymeric ammonium salt is developable with water and exhibits good waterproofness when used in printing plates. However, the composition is still problematic in that its printing durability in flexography is poor, and that, since it contains intragranularly-crosslinked particles, the gel particles still remain as they are in the composition or drop away from the composition after and during its development with an aqueous developer, thereby roughening the relief edges as formed after the development. Specifically, the image reproducibility of the composition for fine relief patterns is poor.